An indexable rotary cutting tool having pluralities of inserts each having cutting edges detachably attached to a tool holder can cut a work with high efficiency. However, when an indexable rotary cutting tool comprising small inserts is used for accurate finishing, chipping and breakage are likely to occur in cutting edges because of insufficient rigidity to stress generated by cutting. Chipping and breakage not only reduce the life of cutting edges, but also deteriorate finished surface quality. Accordingly, various inserts having such shapes as to avoid the chipping and breakage of cutting edges have been proposed.
One means for preventing chipping and breakage is to reduce the vibration of cutting edges. An insert for a cutting tool using such means is disclosed in JP 2003-19617 A. As shown in FIGS. 24(a) and 24(b), this cutting tool insert 201 has a substantially polygonal plate shape comprising a seat-contacting surface 211, an upper surface 212 having a rake face 221A, and flanks 213 between the upper surface 212 and the seat-contacting surface 211; a main cutting edge 216A extending from one corner cutting edge 214A comprising a first main cutting edge 217A, and a second main cutting edge 218A connected to the first main cutting edge 217A with an obtuse angle α; the first main cutting edge 217A being inclined toward the seat-contacting surface 211 with an obtuse angle β to the second main cutting edge 218A; and the first main cutting edge 217A being shorter than the second main cutting edge 218A; the flank 213 comprising a first flank 223A and a second flank 224A; an auxiliary cutting edge 219A extending from the corner cutting edge 214A on the opposite side to the main cutting edge 216A; and the auxiliary cutting edge 219A being connected to the ridgeline 228A with an obtuse angle θ. This shape provides a cutting edge with a large axial rake angle Ax to reduce cutting resistance, thereby reducing vibration. However, JP 2003-19617 A cannot sufficiently prevent chipping and breakage in the corner cutting edge 214A and the first main cutting edge 217A, because (a) a flank of the corner cutting edge 214A is not provided with an angle-changing surface portion having a clearance angle continuously changing along the corner cutting edge 214A, and because (b) the first main cutting edge 217A does not have a receding portion at its end.
JP 8-66815 A discloses, as shown in FIGS. 25(a) to 25(c), a cutting insert 303 for a rotary cutting tool, which comprises a pair of parallel main cutting edges 304a, 304b, a pair of auxiliary cutting edges 305a, 305b laterally extending relative to the main cutting edges 304a, 304b, corner cutting edges 306a, 306b each connecting each main cutting edge 304a, 304b to each auxiliary cutting edge 305a, 305b, rake faces 313, and flanks 321, 322, the cutting edge angle θ of each corner cutting edge 306a, 306b being 50-70°. When the cross-sectional view [FIG. 25(b)] of the main cutting edge is compared with the cross-sectional view [FIG. 25(c)] of the auxiliary cutting edge, the main cutting edge and the auxiliary cutting edge seem to have different clearance angles. However, JP 8-66815 A describes that the clearance angle is substantially constant along the cutting edge, failing to describe a technical idea of making the clearance angle of the auxiliary cutting edge (bottom edge) larger than that of the main cutting edge (peripheral cutting edge). Accordingly, the cutting insert 303 of JP 8-66815 A cannot improve the cutting performance of the peripheral cutting edge while preventing the chipping and breakage of the bottom cutting edge, failing to improve a finished surface condition of a work. Of course, JP 8-66815 A neither discloses nor suggests that the corner cutting edges 306a, 306b are provided with angle-changing surface portions each having a clearance angle continuously changing in a region from the bottom cutting edge to the peripheral cutting edge, to prevent the chipping and breakage of the corner cutting edges 306a, 306b. 